U.S. patent application number 08/946741 was filed with the patent office on 2002-01-03 for graft structures with compliance gradients.
Invention is credited to BERG, TODD ALLEN, GOLDSTEEN, DAVID S..
Application Number | 20020002395 08/946741 |
Document ID | / |
Family ID | 25484918 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020002395 |
Kind Code |
A1 |
BERG, TODD ALLEN ; et
al. |
January 3, 2002 |
GRAFT STRUCTURES WITH COMPLIANCE GRADIENTS
Abstract
A distensible artificial tubular graft structure is provided
that has a compliance gradient. The graft may be used to repair a
patient's body organ tubing. For example, the graft may be used to
replace or supplement portions of a patient's vascular system. The
ends of the graft structure may have compliances that are matched
to the compliances of the body organ tubing to which they are
attached. Distensible compliance-matched connector structures may
be used to attach the graft to the body organ tubing.
Inventors: |
BERG, TODD ALLEN; (LINO
LAKES, MN) ; GOLDSTEEN, DAVID S.; (MINNEAPOLIS,
MN) |
Correspondence
Address: |
G VICTOR TREYZ
FISH AND NEAVE
1251 AVENUE OF THE AMERICAS
NEW YORK
NY
100201104
|
Family ID: |
25484918 |
Appl. No.: |
08/946741 |
Filed: |
October 9, 1997 |
Current U.S.
Class: |
623/1.4 |
Current CPC
Class: |
A61F 2/06 20130101 |
Class at
Publication: |
623/1.4 |
International
Class: |
A61F 002/06 |
Claims
The invention claimed is:
1. A graft for installation in the body of a patient between
portions of body organ tubing with different compliances,
comprising a distensible artificial tubular graft structure with a
compliance gradient along the length of the artificial tubular
graft structure.
2. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure further comprises an end portion with a
compliance that matches the compliance of one of the portions of
body organ tubing.
3. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure further comprises opposing end portions
with compliances that match the respective compliances of the
portions of body organ tubing.
4. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes a tubular frame.
5. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes a flexible tubular nitinol
frame.
6. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises a frame covered with an elastic
coating.
7. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises a flexible tubular nitinol frame
covered with a silicone coating.
8. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes a frame having a density that
varies along the length of the distensible artificial tubular graft
structure, wherein the compliance of the distensible artificial
tubular graft structure is determined at least in part by the
density of the frame.
9. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes a mesh frame having a density that
varies along the length of the distensible artificial tubular graft
structure due at least in part to corresponding variations in the
pattern of the mesh, wherein the compliance of the distensible
artificial tubular graft structure is determined at least in part
by the density of the mesh.
10. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes a mesh frame having a pattern that
varies along the length of the mesh frame, wherein the compliance
of the distensible artificial tubular graft structure is determined
at least in part by the pattern of the mesh.
11. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes a mesh frame having a pic count
that varies along the length of the mesh frame, wherein the
compliance of the distensible artificial tubular graft structure is
determined at least in part by the pic count of the mesh.
12. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises a nitinol wire frame covered with
a silicone coating, the nitinol wire frame having a density that
varies along the length of the frame, wherein the compliance of the
distensible artificial tubular graft structure is determined at
least in part by the density of the nitinol wire frame.
13. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure has at least a first portion having a first
compliance and a second portion having a second compliance
different from the first compliance such that the distensible
artificial tubular graft structure compliance gradient is a stepped
compliance gradient.
14. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure further comprises at least some portions
having a substantially smooth compliance gradient.
15. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises a tubular elastic structure
having a thickness that varies from one end to the other, wherein
the compliance of the distensible artificial tubular graft
structure is determined at least in part by the thickness of the
tubular elastic structure.
16. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises a tubular frame covered with an
elastic coating, the elastic coating having a thickness that varies
from one end to the other, wherein the compliance of the
distensible artificial tubular graft structure is determined at
least in part by the thickness of the elastic coating.
17. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes a radially-compressed
conically-heat-set tubular nitinol frame, wherein the compliance of
the distensible artificial tubular graft structure is determined at
least in part by the degree to which the conically-heat-set tubular
nitinol frame is radially compressed.
18. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes portions with different
durometers, wherein the compliance of the distensible artificial
tubular graft structure is determined at least in part by the
durometers of the portions with different durometers.
19. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure includes portions with different Young's
modulus, wherein the compliance of the distensible artificial
tubular graft structure is determined at least in part by the
Young's modulus of the portions with different Young's modulus.
20. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises: a first graft structure portion
having a first frame portion covered with a first coating having a
first durometer; and a second graft structure portion having a
second frame portion covered with a second coating having a second
durometer different from the first durometer, so that the first and
second graft structure portions have different compliances.
21. The graft defined in claim 1 further comprising connector
structures for attaching the graft between the first and second
portions of the body organ tubing.
22. The graft defined in claim 1 further comprising flexible
connector structures for attaching the graft between the first and
second portions of the body organ tubing.
23. The graft defined in claim 1 further comprising flexible
connector structures for attaching the graft between the first and
second portions of the body organ tubing, wherein each connector
structure has a compliance that is matched to a respective one of
the first and second portions of the body organ tubing.
24. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises a tubular elastic structure
having pores, wherein the compliance of the distensible artificial
tubular graft structure is determined at least in part by the
quantity of the pores in the tubular elastic structure.
25. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises a tubular elastic structure
having pores, wherein the compliance of the distensible artificial
tubular graft structure is determined at least in part by the size
of the pores in the tubular elastic structure.
26. The graft defined in claim 1 wherein the distensible artificial
tubular graft structure comprises a tubular elastic structure
having pores, wherein the compliance of the distensible artificial
tubular graft structure is determined at least in part by the size
and the quantity of the pores in the tubular elastic structure.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to tubular graft structures for
replacing or supplementing a patient's natural body organ tubing.
More particularly, the invention relates to tubular graft
structures in which the elastic compliance of the graft varies
along the length of the graft.
[0002] A patient's weakened or diseased body organ tubing can often
be repaired by replacing or supplementing the patient's existing
natural body organ tubing with an artificial graft structure. One
of the goals in using artificial grafts to repair natural body
organ tubing is to match the characteristics of the artificial
graft to those of the natural graft as closely as possible. For
example, an important property of artificial grafts used to repair
blood vessels is that they be distensible like natural blood
vessels. Distensible grafts are less susceptible to blood clot
formation than other grafts, because distensible grafts pulsate
during blood flow, which tends to hinder blood clot formation. As
described in Goldsteen et al. U.S. patent application Ser. No.
08/839,080, filed Apr. 23, 1997, distensible grafts may be formed
from a nitinol mesh frame covered with a silicone coating.
[0003] The natural distensibility of an artery allows energy to be
stored in the walls of the artery during periods of systolic blood
pressure and allows energy to be released from the walls during
periods of diastolic blood pressure. Storage and subsequent release
of energy by the distensible artery walls helps to sustain blood
flow.
[0004] The distensibility of a given portion of natural body organ
tubing or artificial graft tubing can be quantified by its
compliance, which is defined as the elastic change in diameter of
the tubing per unit fluid pressure inside the tubing. The
compliance of an artery is determined by the amount of elastin
fibers in the artery wall. The downstream or distal portions of the
artery are typically less compliant than the upstream or proximal
portions of the artery. This gradient in the compliance of the
artery allows the upstream portions of the artery to match the
relatively high compliance of vessels in the upstream artery
environment and allows the downstream portions of the artery to
match the lower compliance of the peripheral blood vessel beds fed
by the downstream portions of the artery. Because the compliance of
each portion of the artery is matched to the compliance of the
blood vessels connected to that portion of the artery, stress and
possible damage to the artery walls due to abrupt transitions in
compliance is reduced.
[0005] It is therefore an object of the present invention to
provide a distensible artificial graft having compliance properties
similar to the compliance properties of the natural body organ
tubing of a patient.
[0006] It is also an object of the present invention to provide a
distensible artificial graft that has a compliance gradient and is
compliance matched to the body organ tubing of a patient.
SUMMARY OF THE INVENTION
[0007] These and other objects of the invention are accomplished in
accordance with the principles of the present invention by
providing a distensible artificial graft that may be used to
replace or supplement diseased or damaged natural body organ
tubing. For example, the graft may be used to repair blocked blood
vessels. Because the graft is distensible, in vascular applications
the graft pulsates like natural blood vessels, which may reduce the
incidence of blood clot formation.
[0008] The graft has a compliance (i.e., change in diameter of the
graft per unit pressure inside the graft) that varies along the
length of the graft. This compliance gradient allows the graft to
create a smooth transition between portions of body organ tubing
with different compliances. For example, the graft may be used to
connect an upstream portion of an artery (which has a relatively
high compliance) with a downstream portion of the artery (which has
a relatively low compliance). By matching the magnitude of the
compliance at each end of the graft with the portion of body organ
tubing to which it is connected, abrupt transitions in compliance
are avoided. Avoiding such abrupt transitions reduces stress and
possible damage to the body organ tubing in the vicinity of the
graft.
[0009] The graft may be formed from any suitable distensible
tubular structure in which compliance can be varied along the
length of the structure. For example, the graft may be formed from
a flexible tubular mesh frame covered with an elastic coating. A
suitable mesh may be formed from nitinol wire. A suitable coating
is silicone.
[0010] The compliance gradient may be formed by varying the density
of the mesh along the length of the graft. Higher density mesh is
generally less compliant than lower density mesh. Mesh density can
be controlled during graft fabrication by varying the pattern of
the mesh. For example, a tighter weave or braid increases the
density of the mesh. Preferably, the density of the mesh is
controlled by varying the pic count of the mesh. Other techniques
that may be used to control the density of the mesh include varying
the size of the nitinol wire and varying the number of wire strands
that are used to form the mesh.
[0011] If desired, the compliance gradient may be formed by varying
the thickness of the elastic coating used to cover the frame.
Portions of the graft where the coating is thick are less compliant
than portions of the graft where the coating is thin. If the graft
is formed primarily from a single material (e.g., a polymeric
substance), the graft compliance can be controlled by varying the
thickness of the material.
[0012] A compliance gradient may be created by compressing a
conical frame into a cylindrical graft shape. The conical frame may
be formed on a conical mandrel. If a heat sensitive memory-effect
metal such as nitinol is used as the frame material, the frame may
initially be formed in a cylindrical shape and subsequently
stretched and heat-set in the desired conical shape. After the
conical frame shape is created, the frame is radially compressed
into a cylindrical shape and covered with a suitable coating such
as silicone. The portions of the frame that were the largest
radially before compression contribute a radial outward bias to the
completed graft structure. The outward bias of such frame portions
increase the compliance of the corresponding portions of the
graft.
[0013] Another way in which to create the compliance gradient for
the graft is to vary the properties of the materials used to form
the graft. For example, coatings of different durometer or Young's
modulus may be used to cover different portions of a frame
structure. If desired, the porosity of the graft may be varied to
create the compliance gradient.
[0014] Distensible connector structures may be used to attach the
graft to the body organ tubing. One suitable distensible connector
structure is an elastic ring with radially extending barbs or
hooks. When the graft is installed in the patient, the elastic ring
expands to force the barbs through the graft and into the body
organ tubing, thereby attaching the graft to the body organ tubing.
If desired, the compliance of such connector structures can be
matched to the compliance of the body organ tubing at the
attachment site.
[0015] Further features of the invention, its nature and various
advantages will be more apparent from the accompanying drawings and
the following detailed description of the preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a partially cut-away perspective view of body
organ tubing in which a section of tubing has been replaced by a
graft in accordance with the present invention.
[0017] FIG. 2 is a graph of compliance plotted versus distance
along the longitudinal axis of a prior art graft structure.
[0018] FIGS. 3a and 3b are and graphs of compliance plotted versus
distance along the longitudinal axis of graft structures in
accordance with the present invention.
[0019] FIGS. 4a-d are side views of various graft structures in
accordance with the present invention in which the compliance of
the graft varies as a function of distance along the longitudinal
axis of the graft.
[0020] FIGS. 5a and 5b are side views of additional graft
structures in accordance with the present invention in which the
compliance of the graft varies as a function of distance along the
longitudinal axis of the graft.
[0021] FIG. 6a is a side view of an illustrative graft structure in
accordance with the present invention in which compliance is
controlled by varying the pore size of the graft structure.
[0022] FIG. 6b is a graph showing the relationship between pore
size and compliance (length) for the graft of FIG. 6a.
[0023] FIG. 7a is a side view of an illustrative graft structure in
accordance with the present invention in which compliance is
controlled by varying the quantity of pores in the graft
structure.
[0024] FIG. 7b is a graph showing the relationship between pore
quantity and compliance (length) for the graft of FIG. 7a.
[0025] FIG. 8 is a perspective view of a graft structure showing
illustrative distensible graft connector structures in accordance
with the present invention that are used to connect the graft to
natural body organ tubing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] An illustrative distensible artificial graft 10 in
accordance with the present invention is shown in FIG. 1. Graft 10
may be a structure formed from a flexible coating 12 covering a
frame 14. The preferred materials for forming frame 14 of graft 10
are metals, although polymeric materials may also be used. The
presently most preferred material is a braid of nitinol wire.
Coating 12 is preferably an elastic bio-compatible material such as
silicone, which fills the apertures formed by the wires in frame
14. Other materials that may be used for coating 12 include
polymeric materials such as stretchable urethane, stretchable
polytetrafluoroethylene (PTFE), natural rubber, and the like.
[0027] If desired, coating 12 can be formed with microscopic pores
to help improve bio-compatibility. A preferred method of providing
a desired porosity is to make coating 12 from an elastic material
that is mixed with particles of a material that can be removed
(e.g., by vaporization) after coating 12 has been applied to frame
14. When the particles are removed, voids are left in coating 12
that give it porosity. The degree of porosity of coating 12
influences its elasticity, so the compliance of coating 12 may be
controlled by varying the porosity of coating 12.
[0028] If desired, graft 10 may be provided with additional
coatings such as medicated coatings, hydrophilic coatings,
smoothing coatings, collagen coatings, human cell seeding coatings,
etc., as described in the above-mentioned Goldsteen et al. U.S.
patent application No. 08/745,618, filed Nov. 7, 1996, which is
hereby incorporated by reference herein in its entirety. The
above-described preferred porosity of coating 12 helps graft 10 to
retain these coatings.
[0029] In the illustrative example of FIG. 1, graft 10 has been
used to replace a section of body organ tubing between body organ
tubing 16 and body organ tubing 18. Body organ tubing 16 and 18
appears elongated in FIG. 1, but graft 10 may also be used to
connect body organ tubing of any suitable shape. As defined herein,
the term "body organ tubing" generally refers to elongated
fluid-containing body organ tissues such as blood vessels and the
like and to similar but less elongated body organ tissue structures
such as portions of the heart wall. Body organ tubing may be
vascular tubing or any other type of body organ tubing.
[0030] In accordance with the present invention, the compliance of
distensible graft 10 at end 20 is matched to the compliance of body
organ tubing 16 at end 22. In addition, the compliance of graft 10
at end 24 is matched to the compliance of body organ tubing 18 at
end 26. Repairs of the type shown in FIG. 1 can be made to any
desired type of body organ tubing, but compliance matching is
particularly important in blood vessel repairs to reduce stress due
to abrupt transitions in compliance between ends 20 and 22 and
between 24 and 26.
[0031] In conventional graft arrangements, the compliances of body
organ tubing and grafts are not matched. As shown in FIG. 2, the
compliance of the graft of region II does not match the compliance
of the body organ tubes of regions I and III at transitions 28 and
30. In part, the abruptness of transitions 28 and 30 is due to the
relatively low compliance of the conventional graft of region II.
The abruptness of transitions 28 and 30 is also exacerbated by the
mismatch between the gradients of the body organ tubing compliances
of regions I and III and the lack of any gradient in the compliance
of the graft in region II.
[0032] With the arrangement of the present invention, the magnitude
and the gradient of the compliance of graft 10 (FIG. 1) may be
matched to the magnitude and gradient of the compliance of the body
organ tubing section that was replaced by graft 10, as shown by
graft compliance curve 32. The graft compliance at end 38 of curve
32 is matched with the body organ tubing compliance at end 40 of
curve 34 and the graft compliance at end 42 of curve 32 is matched
with the body organ tubing compliance at end 44 of curve 36.
Matching the compliance gradient and the compliances of the ends of
graft 10 with the compliances of the respective ends of the body
organ tubing reduces stress and possible damage to the body organ
tubing that might otherwise result using a conventional arrangement
such as shown in FIG. 2.
[0033] It is not necessary for the match between the compliance
gradient and compliance at the ends of graft 10 and the ends of the
body organ tubing to be perfect. For example, a suitable graft 10
might have the compliance shown by graft compliance curve 46.
Although the match of the graft of curve 46 is not as good as the
graft of curve 32, the graft of compliance curve 46 is
significantly better at reducing stress and possible body organ
tubing damage due to abrupt transitions than the conventional graft
of FIG. 2.
[0034] Grafts having compliances such as those shown by curves 32
and 46 are suitable for repairing sections of body organ tubing
having the compliances of curves 34 and 36. Moreover, the smooth
monotonic gradient of the compliances of curves 32 and 46 avoids
abrupt transitions in compliance within graft 10 (FIG. 1) and
optimizes the hemodynamics of graft 10.
[0035] If desired, grafts may be formed that have less smooth
compliance gradients than those shown in FIG. 3a. For example, the
magnitude and the gradient of the compliance of graft 10 of FIG. 1
may be as shown in FIG. 3b. In compliance curve 50 of FIG. 3b, the
magnitude of the graft compliance in portion 52 is at a first level
and the graft compliance in portion 54 is at a second level.
Nevertheless, the graft compliance at end 56 matches the body organ
compliance at end 58 and the graft compliance at end 60 matches the
body organ tubing compliance at end 62. Matching the compliances of
the ends of the graft of curve 50 with the compliances of the
respective ends of the body organ tubing reduces stress and
possible damage to the body organ tubing in the vicinity of the
transitions between the body organ tubing and graft. Although there
is a transition in the compliance level in the center of the graft
of curve 50, a transition in that location is generally less likely
to cause tissue damage than a comparable transition at a connection
(anastomosis) between the graft and body organ tubing.
[0036] The compliance profiles of FIGS. 3a and 3b are illustrative
only. Other compliance profiles may be used if desired. In general,
the compliance of an off-the-shelf graft will not be perfectly
matched to the compliance of a given section of body organ tubing
to be repaired. However, the grafts of the present invention
preferably have compliance gradients and compliance magnitudes at
their ends that match the body organ tubing to which they are
connected well enough to reduce the stress and potential body organ
tubing damage that may result using conventional grafts.
[0037] Various techniques may be used to form graft structures with
compliance gradients in accordance with the present invention. A
number of illustrative structures are shown in FIGS. 4a-d. As shown
in FIG. 4a, a graft with a compliance gradient may be formed by
varying the density of the metal mesh used to form frame 62. The
density of frame 62 varies as a function of the distance along the
longitudinal axis of frame 62. The density of frame 62 is higher at
end 64 than at end 66, so the compliance of the graft formed using
frame 62 is greater at end 66 (where it is relatively easier to
radially expand the graft) than at end 64 (where it is relatively
more difficult to radially expand the graft). If frame 62 is a
formed from metal wire, the density (and therefore the compliance)
of frame 62 is preferably varied by changing the pic count (the
number of wire intersections per inch along a single longitudinally
oriented line on the surface of frame 62) along the length of frame
62. The density and compliance of frame 62 may also be varied by
changing the strand count (e.g., 16, 32, or 64, etc. or by changing
the diameter of the wire in the frame. Compliance may also be
varied by changing the pattern of weave or braid that is used to
form the mesh frame. Tightly woven or braided patterns generally
have lower compliances than loosely woven or braided patterns.
After being formed with a compliance gradient, frame 62 may be
covered with a coating such as coating 12 of FIG. 1.
[0038] If desired, the compliance gradient may be formed by varying
the thickness of elastic coating 12. As shown in FIG. 4b, frame 68
is covered with coating 70, which is relatively thinner at end 72
and relatively thicker at end 74. As a result, the compliance of
graft 76 is greater at end 72 (where the thin coating makes it
relatively easier to radially expand the graft) than at end 74
(where the thick coating makes it relatively difficult to radially
expand the graft).
[0039] If the graft is formed from a flexible polymer or other
suitable elastic material without an internal frame, the thickness
of the polymer can be varied as a function of the distance along
the longitudinal axis of the graft. As shown in FIG. 4c, such a
graft 78 has a compliance gradient, because the compliance at end
80 (where thin graft wall 82 makes it relatively easier to radially
expand the graft) is more than the compliance at end 84 (where
thick graft wall 86 makes it relatively difficult to radially
expand the graft).
[0040] Another technique for creating a graft with a compliance
gradient involves using a frame formed from a heat sensitive metal
such as nitinol. As shown in FIG. 4d, nitinol frame 88 is initially
formed in conical shape 90 by directly weaving or braiding frame 88
into that shape or by stretching a cylindrical mesh into conical
shape 90 and setting shape 90 with a heat treatment. The
conically-heat-set frame in shape 90 is then forced to assume shape
92 (e.g., by radially compressing frame 88 within a cylindrical
tube). A coating such as coating 12 is applied to frame 88 while
frame 88 has shape 92, thereby forming graft 94. At end 96, the
compressed frame 88 desires to expand radially outward to regain
uncompressed shape 90, so end 96 is prestressed for radial
expansion. At end 98, frame 88 is already nearly in uncompressed
shape 90, so there is relatively little radial expansion
prestressing. Graft 94 therefore has a compliance gradient, because
the compliance of graft 94 is higher near end 96 than near end
98.
[0041] The compliance gradients of the grafts of FIGS. 4a-d are
relatively smooth and continuous, such as shown by graft compliance
curves 32 and 46 of FIG. 3a. Smooth gradients are desirable because
they optimize the hemodynamics of the graft. If other techniques
are used to form the graft, compliance gradients such as the
two-level stepped compliance gradient of FIG. 3b can be obtained.
Although the hemodynamics of a graft with a stepped compliance
gradient may not be as optimum as the hemodynamics provided by a
graft with a smooth compliance gradient, the performance of such
grafts may be satisfactory. Grafts with stepped compliance
gradients may also be easier to fabricate in some cases than grafts
with smooth compliance gradients.
[0042] Grafts with stepped compliance gradients may be formed using
a variety of techniques. As shown in FIG. 5a, graft frame 100 may
be formed with different densities. Frame portion 102 may have a
lower density (and therefore higher compliance) than frame portion
104. If frame 100 is a formed from metal wire, the compliance of
frame 100 is preferably varied by changing the pic count (the
number of wire intersections per inch along a single longitudinally
oriented line on the surface of frame 100) used for portions 102
and 104. Portions 102 and 104 may also be formed with different
compliances by changing the pattern of weave or braid that is used
to form the mesh frame, or changing the diameter of the wire in the
frame. After forming frame 100 with the two-step compliance pattern
shown in FIG. 5a, frame 100 may be covered with a coating such as
coating 12 of FIG. 1 to complete the graft.
[0043] If desired, a stepped compliance pattern may be formed by
stretching or compressing the frame and heat-setting the frame, as
described in connection with FIG. 4d.
[0044] Another technique for forming a graft with a stepped
compliance pattern involves varying the compliance of the graft by
varying the properties of the graft coating. As shown in FIG. 5b,
end 106 of frame 108 is covered with coating 110 and end 112 of
frame 108 is covered with coating 114. The compliance of ends 106
and 112 will generally differ depending on the respective material
properties (e.g., durometer, Young's modulus, etc.) of coatings 110
and 112. If desired, multiple layers of coatings may be provided to
vary the compliance of the graft. Smooth graft compliance profiles
(such as shown by curves 32 and 46 of FIG. 3a) may be obtained by
smoothly varying the properties and the number of layers of graft
coating that are used.
[0045] If desired, the compliance of the graft may be varied by
controlling the size and/or quantity of pores in the graft. This is
illustrated in FIGS. 6 and 7.
[0046] As shown in FIG. 6a, graft 310 may be provided with larger
pores 302 in region II than in region I and larger pores in region
III than in region II. This creates a compliance profile such as
shown in FIG. 6b.
[0047] As shown in FIG. 7a, graft 310 may be provided with more
pores 302 in region II than in region I and more pores 302 in
region III than in region II. This creates a compliance profile
such as shown in FIG. 7b. The distribution of pore quantities and
pore sizes may be continuous or step-like. Both the pore size and
pore quantity can be varied if desired.
[0048] Porous graft structures such as grafts 310 of FIGS. 6 and 7
may be formed using a coating made of an elastic material that is
mixed with particles of a material that can be removed (e.g., by
vaporization) after the coating has been applied to a frame (e.g.,
by spraying).
[0049] A number of different connector structures may be used to
install grafts such as graft 10 (FIG. 1). For example, connector
structures 116 of FIG. 8, which are formed from elastic rings with
barbs 118, may be used to connect graft 210 to body organ tubing
such as body organ tubing 16 and 18 of FIG. 1. Ring structures may
be formed of any suitable material, such as an elastic polymer.
Installation may be intraluminally (e.g., by radially compressing
and delivering the grafts through the existing vascular system of
the patient) or may use general surgical techniques. During
installation of graft 210, connector structures 116 may be radially
compressed, so that the ends of graft 210 may be inserted inside
the corresponding ends of the body organ tubing to which graft 210
is to be attached. Once the ends of graft 210 have been positioned
properly for graft attachment, connector structures 116 are
released, which causes barbs 118 to penetrate the surrounding body
organ tubing and thereby hold graft 210 in place.
[0050] Other suitable connector structures include serpentine wire
structures, structures without barbs or hooks, etc. Compliant
connector structures that may be used include the connector
structures described in Berg et al. U.S. patent application Ser.
No.______, filed ______(Case 293/018), and Bachinski U.S. patent
application Ser. No.______, filed (Case 293/023).
[0051] The compliances of the connector structures such as 116 that
are used to install graft 10 (FIG. 1) are preferably matched to the
compliances of the graft ends and the compliances of the body organ
tubing to which the graft is attached. The compliances of connector
structures 116 can be varied by controlling material parameters
such as the durometer and Young's modulus of structures 116 (if the
structures are elastomeric) or by varying the wire density, etc.
(if the structures are formed from wire). Because the connector
structures are relatively short along the longitudinal dimension of
the graft, using such a connector structure will not greatly affect
the overall compliance profile of the graft. Accordingly, although
flexible structures with matched compliances are preferred,
satisfactory results may be obtained using relatively inflexible
connector structures if desired.
[0052] In order to match the compliance of a given graft to the
body organ tubing that is to be repaired, the physician making the
repair may asses the size of the body organ tubing being replaced,
the particular locations in the body to which the graft ends are to
be connected, and the graft length. Grafts with various compliance
profiles are preferably made available to the physician, so that
the physician may select a graft that matches the needs of the
patient (e.g., age, degree of disease, type of disease, etc.).
[0053] If desired, tubular grafts with compliance gradients may be
formed that have T-shapes or Y-shapes. All such grafts are herein
collectively referred to as "tubular graft structures."
[0054] It will be understood that the foregoing is only
illustrative of the principles of the invention, and that various
modifications can be made by those skilled in the art without
departing from the scope and spirit of the invention.
* * * * *